Fix build of DistributedMutex

Summary:
It was broken, fix it.

Fixes https://github.com/facebook/folly/issues/1092

Reviewed By: yfeldblum

Differential Revision: D14842180

fbshipit-source-id: 899c95fdac3622c78c9c6f326cd14898007ddf6c

folly/synchronization/DistributedMutex-inl.h

@@ -206,7 +206,7 @@ using CombineFunction = detail::InlineFunctionRef<void(), 48>;
 template <template <typename> class Atomic>
 class Waiter {
  public:
-  Waiter() = default;
+  Waiter() {}
   Waiter(Waiter&&) = delete;
   Waiter(const Waiter&) = delete;
   Waiter& operator=(Waiter&&) = delete;